The Compton spectrometer and imager (COSI) is an upcoming NASA space telescope in the MeV range. COSI's primary science goals include precisely mapping positron annihilation and nuclear line emissions in the Milky Way galaxy through Compton imaging. This relies on our ability to maintain COSI's spectral performance over its mission lifetime. The changes in the detectors' gain characteristics over time will result in inaccurate measurement of astrophysical gamma-ray energies. Moreover, observations from past MeV telescopes and proton-beam experiments have shown that radiation damage in space causes photopeak shifts and spectral line broadening. These necessitate a plan for regular, in-orbit spectral calibration. In this study, we demonstrate a method to monitor and recalibrate the COSI detectors using background line emissions produced by the space radiation environment. We employ Monte Carlo simulations of particle background and show that strong background lines arise from nuclear excitation of COSI's detectors (germanium) and cryostat (aluminum) materials. These span COSI's entire bandwidth for single-site interactions and can be used to monitor the effects of radiation damage and gain shifts every 12 h at the full instrument level and every 16 days at the individual detector level. Methods developed by Pike et al. (2023, 2025) to correct the effects of hole trapping and gain characteristics can then be applied to recover the original spectral performance. These results inform COSI's telemetry requirements for calibration and housekeeping data and rule out the need for an onboard radioactive calibration source, which would have increased the complexity of the spacecraft.

We present two NuSTAR observations of the X-ray transient, MAXI J1752-457, following a superburst that was observed by MAXI/Gas Slit Camera (GSC) in 2024 November. NuSTAR follow-up confirmed that MAXI J1752-457 is coincident with the previously observed Einstein Probe source, EP240809a. We performed a spectral analysis of the source during both NuSTAR observations, and we find that the hard X-ray spectra are consistent with the inclusion of a spherical blackbody component and a steep, nonthermal power-law component. At about 79 hr after the onset of the superburst, we find a blackbody temperature of kTbb = 0.60 +/- 0.1 keV and Rbb/D8=6.0-0.3+0.4 km (not including corrections for scattering in the neutron star atmosphere), where D8 is the source distance, which is not yet known, in units of 8 kpc. We found that the blackbody temperature did not change significantly in the 1 day interval between successive NuSTAR observations, and we performed an energy-resolved timing analysis that showed that the source variability was dominated by red noise in the power-law component, suggesting coupling with an accretion disk. We infer that the source had entered an accretion-powered flux state. Furthermore, we measure a photon index of Gamma approximate to 4, much steeper than those typically observed during accretion onto neutron stars at similar luminosities. This is suggestive of ongoing evolution of the electron energy distribution responsible for the power-law component several days after the superburst, but the lack of hard X-ray observations prior to and throughout the superburst makes it difficult to present a conclusive physical interpretation of this result.

The energy range from a few hundred keV to a few MeV includes important probes such as nuclear gamma-rays and the 511 keV annihilation line. However, compared to X-rays and GeV/TeV gamma-rays, this range suffers from lower sensitivity by orders of magnitude. The upcoming NASA SMEX satellite mission Compton Spectrometer and Imager (COSI), scheduled for launch in 2027, is expected to break through this limitation with its Compton telescope utilizing a germanium semiconductor detector, covering the 0.2-5 MeV energy range. In addition to the main instrument, two Background and Transient Observer (BTO) detectors will be installed on COSI. The detectors are NaI(Tl) scintillators coupled with SiPMs, and they are being developed as a student collaboration project. BTO aims to 1) measure background radiation in orbit to maximize COSI's sensitivity and 2) detect GRBs and other gamma-ray transients. For this purpose, it is required to cover the lower-energy range from 30 keV to 2 MeV with < 20

The central 2 × 0.8 deg ^2 region of our Galaxy contains ∼10,000 X-ray point sources that were detected by a series of Chandra observations over the last two decades. However, the limited bandpass of Chandra below 8 keV hampered their spectroscopic classification. In 2016, the initial NuSTAR Galactic center (GC) survey detected 77 X-ray sources above 10 keV. The hard X-ray detections indicate magnetic cataclysmic variables, low-mass X-ray binaries, high-mass X-ray binaries (HMXBs), or even pulsars. The possibility of HMXB detections is particularly interesting given the dearth of identified HMXBs in the GC. We conducted a search for bright ( K _s ≲ 16 mag) near-infrared (NIR) counterparts to the hard X-ray sources—utilizing their Chandra positions—in order to identify HMXB candidates. We identified seven NuSTAR sources with NIR counterpart candidates whose magnitudes are consistent with HMXBs at the GC. We assessed the likelihood of random association for these seven sources, and determined that two have a nonrandom association with a probability exceeding 99.98%, making them strong HMXB candidates. We analyzed broadband NuSTAR, Chandra, and XMM-Newton spectral data for these two candidates, one of which was previously identified as a red supergiant. We find that the X-ray spectra are consistent with HMXBs. If confirmed through follow-up NIR spectroscopic studies, our findings will open a new window into our understanding of the intrinsic luminosity distribution of HMXBs in our Galaxy in general and the GC HMXB population in particular.

The Compton Spectrometer and Imager balloon payload (COSI-Balloon) is a wide field-of-view Compton gamma-ray telescope that operates in the 0.2–5 MeV bandpass. COSI-Balloon had a successful 46 day flight in 2016 during which the instrument observed the Crab Nebula, Cygnus X-1, and Centaurus A. Using the data collected by the COSI-Balloon instrument during this flight, we present the source flux extraction of signals from the variable balloon background environment and produce images of these background-dominated sources by performing Richardson–Lucy deconvolutions. We also present the spectra measured by the COSI-Balloon instrument, compare and combine them with measurements from other instruments, and fit the data. The Crab Nebula was observed by COSI-Balloon, and we obtain a measured flux in the energy band 325–480 keV of (4.5 ± 1.6) × 10 3 ph cm −2 s −1 . The model that best fits the COSI-Balloon data combined with measurements from NuSTAR and Swift-BAT is a broken power law with a measured photon index Γ = 2.20 ± 0.02 above the 43 keV break. Cygnus X-1 was also observed during this flight, and we obtain a measured flux of (1.4 ± 0.2) × 10 3 ph cm −2 s −1 in the same energy band and a best-fit result (including data from NuSTAR, Swift-BAT, and INTEGRAL/IBIS) was a cutoff power law with a high-energy cutoff energy of 138.3 ± 1.0 keV and a photon index of Γ = 1.358 ± 0.002. Finally, we present the measured spectrum of Centaurus A and our best model fit to a power law with a photon index of Γ = 1.73 ± 0.01.

The spin of a black hole (BH) encodes information about its formation and evolution history. Yet the understanding of the distribution of BH spins in X-ray binaries (XBs), of the models used to measure spin, and of their impact on systematic uncertainties remains incomplete. In this work, we expand on previous analyses of the entire NuSTAR archive of accreting BH XBs. Prior work compiled a sample of 245 spectral fits using the relativistic reflection method for NuSTAR observations of 36 BH systems. Here, we aim to probe two aspects: the connection between BH spin and binary system properties, and the relationships between parameters in the spectral fits. We identify moderate negative correlations between spin uncertainty and both BH mass and system inclination, and a moderate positive correlation with distance. We also point out tentative multidimensional degeneracies between inclination, disk density, Fe abundance, ionization, and the presence or absence of absorption features from ionized outflows linked to disk winds. Lastly, we provide a comprehensive view of the observed distribution of BH spins in XBs, in comparison to spins inferred from gravitational waves. We find that the distribution of BH spins in XBs can be described by a beta distribution with α=5.66 and β=1.09. This data set is highly complex, and the analysis presented here does not fully explore all potential parameter correlations. We make the full data set available in Zenodo to the community to encourage further exploration.

We present mid-infrared (MIR) spectral-timing measurements of the prototypical Galactic microquasar GRS 1915+105. The source was observed with the Mid-Infrared Instrument (MIRI) onboard JWST in June 2023 at an MIR luminosity LMIR approximate to 1036 erg s-1 exceeding past infrared levels by about a factor of 10. In contrast, the X-ray flux is much fainter than the historical average, in the source's now-persistent 'obscured' state. The MIRI low-resolution spectrum shows a plethora of emission lines, the strongest of which are consistent with recombination in the hydrogen Pfund (Pf) series and higher. Low-amplitude (similar to 1 per cent) but highly significant peak-to-peak photometric variability is found on time-scales of similar to 1000 s. The brightest Pf (6-5) emission line lags the continuum. Though difficult to constrain accurately, this lag is commensurate with light-travel time-scales across the outer accretion disc or with expected recombination time-scales inferred from emission-line diagnostics. Using the emission line as a bolometric indicator suggests a moderate (similar to 5-30 per cent Eddington) intrinsic accretion rate. Multiwavelength monitoring shows that JWST caught the source close in time to unprecedentedly bright MIR and radio long-term flaring. Assuming a thermal bremsstrahlung origin for the MIRI continuum suggests an unsustainably high mass-loss rate during this time unless the wind remains bound, though other possible origins cannot be ruled out. Polycyclic aromatic hydrocarbon features previously detected with Spitzer are now less clear in the MIRI data, arguing for possible destruction of dust in the interim. These results provide a preview of new parameter space for exploring MIR spectral timing in X-ray binaries and other variable cosmic sources on rapid time-scales.

This work presents comprehensive multifrequency radio monitoring of the black hole low-mass X-ray binary (LMXB) Swift J1727.8−1613, which underwent its first recorded outburst after its discovery in 2023 August. Through a considerable community effort, we have coalesced the data from multiple, distinct observing programs; the light curves include ∼10 months and 197 epochs of monitoring from seven radio facilities with observing frequencies ranging from (approximately) 0.3–230 GHz. The primary purpose of this work is to provide the broader astronomical community with these light curves to assist with the interpretation of other observing campaigns, particularly nonradio observing frequencies. We discuss the phenomenological evolution of the source, which included (i) multiple radio flares consistent with the launching of discrete jet ejections, the brightest of which reached ∼1 Jy; (ii) temporally evolving radio spectral indices ( α ), reaching values steeper than expected for optically thin synchrotron emission ( α < −1) and emission with significant radiative cooling ( α < −1.5). We have published a digital copy of the data and intend for this work to set a precedent for the community to continue releasing comprehensive radio light curves of future LMXB outbursts.

Astrophysical observations of the cosmos allow us to probe extreme physics and answer foundational questions on our universe. Modern astronomy is increasingly operating under a holistic approach, probing the same question with multiple diagnostics including how sources vary over time, how they appear across the electromagnetic spectrum, and through their other signatures, including gravitational waves, neutrinos, cosmic rays, and dust on Earth. Astrophysical observations are now reaching the point where approximate physics models are insufficient. Key sources of interest are explosive transients, whose understanding requires multidisciplinary studies at the intersection of astrophysics, gravity, nuclear science, plasma physics, fluid dynamics and turbulence, computation, particle physics, atomic, molecular, and optical science, condensed matter and materials science, radiation transport, and high energy density physics. This white paper provides an overview of the major scientific advances that lay at the intersection of physics and astronomy and are best probed through time-domain and multimessenger astrophysics, an exploration of how multidisciplinary science can be fostered, and introductory descriptions of the relevant scientific disciplines and key astrophysical sources of interest.

The Compton Spectrometer and Imager (COSI) is a gamma-ray survey telescope utilizing a compact Compton imager design, enabled by an array of 16 high-resolution germanium cross-strip detectors. After its launch into an equatorial Low Earth Orbit (LEO) in 2027, COSI will experience radiation damage primarily due to energetic protons, with the proton fluence dominated by the passage of COSI through the edge of the South Atlantic Anomaly (SAA) for a few minutes each orbit. We have developed a comprehensive program focused on the modeling, characterization, data correction, and physical repair of radiation damage effects in the COSI detectors. We have performed energetic proton beam irradiations of a spare COSI detector at a proton synchrotron, with proton fluences consistent with multiple years of exposure to the COSI space radiation environment. These exposures allow us to characterize the relationship between proton fluence and induced charge trapping. We demonstrate our techniques to correct for trapping effects, as well as characterize the effectiveness of high-temperature annealing on correcting this damage, as characterized by the resulting spectral performance of the detector. We will present our efforts to characterize the effects of radiation damage in the COSI detectors, as well as our techniques for correcting these effects in the data analysis pipeline and ultimately repairing the detectors on orbit every few years through high-temperature annealing.

For the first time in nearly a decade, a bright transient was detected in the central parsec (pc) of the Galaxy. MAXI J1744-294, or – as it was known in its previous life – Swift J174540.2-290037, was discovered in outburst by the MAXI telescope in January 2025. We present the results of a broadband, multi-wavelength study of MAXI J1744-294, including data from the NuSTAR, Chandra, XMM-Newton, Swift, and NICER X-ray telescopes, as well as complementary radio and near-infrared observations. We analyze the changing X-ray emission as the outburst evolved from the high/soft to the low/hard state. Using relativistic reflection features in the data, we estimate a spin of a>0.92 and viewing inclination θ=28^+3_-4 deg, consistent with the parameters measured for Swift J174540.2-290037. Based on the spectral and temporal characteristics of MAXI J1744-294, we reaffirm its classification as a candidate black hole (BH) low-mass X-ray binary (LMXB) – the third candidate BH transient discovered within 20 arcsec of the Galactic supermassive black hole Sgr A*. This work provides further evidence for a cusp of BH-LMXBs in the central pc of our Galaxy, as argued for in previous observational studies and suggested by analytical and theoretical work. Our ongoing multi-wavelength study, involving a complementary range of observatories and spanning different outburst states, can serve as a model for future time domain astrophysics research.

We present a modified Richardson-Lucy (RL) algorithm tailored for image reconstruction in MeV gamma-ray observations, focusing on its application to the upcoming Compton Spectrometer and Imager (COSI) mission. Our method addresses key challenges in MeV gamma-ray astronomy by incorporating Bayesian priors for sparseness and smoothness while optimizing background components simultaneously. We introduce a novel sparsity term suitable for Poisson-sampled data in addition to a smoothness prior, allowing for flexible reconstruction of both point sources and extended emission. The performance of the algorithm is evaluated using simulated three-month COSI observations of gamma-ray lines of $^{44}$Ti (1.157 MeV), $^{26}$Al (1.809 MeV), and positron annihilation (0.511 MeV), respectively, representing various spatial features. Our results demonstrate significant improvements over conventional RL methods, particularly in suppressing artificial structures in point source reconstructions and retaining diffuse spatial structures. This work represents an important step towards establishing a robust data analysis for studying nucleosynthesis, positron annihilation, and other high-energy phenomena in our Galaxy.

The Compton Spectrometer and Imager (COSI) is a NASA Small Explorer (SMEX) satellite mission scheduled to launch in 2027 into a low-Earth equatorial orbit. COSI is a Compton telescope composed of 16 high-purity germanium cross-strip detectors, which are surrounded on the bottom and sides by a BGO anti-coincidence system to veto radiation from Earth's atmosphere. COSI will operate in the energy band from 0.2 to 5.0 MeV. Its germanium detectors enable excellent energy resolution, all-sky imaging due to its instantaneous field of view covering 25% of the sky, and the observation of polarization. COSI's main science goals include uncovering the origins of Galactic positrons, revealing sites of Galactic nucleosynthesis, gaining insights into extreme environments such as those near black holes and pulsars through polarization, and probing the physics of multi-messenger events such as merger events.

Next-generation gamma-ray observatories aim to enable precision measurements in high-energy astrophysics using advanced semiconductor detector technologies. Meeting the scientific requirements of modern instruments demands detector systems that provide high spatial and spectral resolution across large detection areas, with strict limits on power consumption and mass. These needs drive innovation in front-end electronics and mixed-signal processing to support compact detector electrode geometries. Application-specific integrated circuits (ASICs) are essential in front-end readout electronics, enabling high-channel-density and low-power systems, while maintaining low-noise performance suitable for space-based instruments and balloon-borne payloads. The NRL4 (Naval Research Laboratory 4) is a recently developed 32-channel front-end ASIC featuring low-power, low-noise channels consisting of charge-sensitive preamplifiers, 4 configurable gain settings, dual configurable shapers for optimized timing and energy resolution, trimmable per-channel discrimination, time-to-analog conversion, and peak-detect output. The NRL4 has been integrated with a high-purity germanium (HPGe) dual-sided strip detector with a 1.16 mm strip pitch. Energy resolution of 3 keV full width at half maximum (FWHM) at 59.54 keV was achieved with a gain of 18.4 mV/fC and a slow shaper peaking time of 2 μs. Preliminary results from ongoing research demonstrate the suitability of the NRL4 for high-resolution, low-power gamma-ray spectroscopy for ground and space-based missions.

The Compton Spectrometer and Imager (COSI) is an upcoming NASA Small Explorer satellite mission, designed for all-sky observations in the soft gamma-ray domain with the use of germanium detectors (GeDs). An active Anticoincidence System (ACS) of BGO scintillators surrounds the GeDs to reduce the background and contribute to the detection of transient events. Accurately modeling the ACS performance requires simulating the intricate scintillation processes within the shields, which significantly increases the computational cost. We have encoded these effects into a correction matrix derived from dedicated Geant4 simulations with the inclusion of the optical physics. For this purpose, we use laboratory measurements for the energy and spatial response of the ACS lateral wall to benchmark the simulation and define instrument parameters, including the BGO absorption length and the electronic noise. We demonstrate that the simulations replicate the experimental energy resolution and light collection uniformity along the BGO crystal, with maximum discrepancies of 20% and 10%, respectively. The validated simulations are then used to develop the correction matrix for the lateral wall, accounting for the light collection efficiency and energy resolution based on the position within the crystal. The gamma-ray quantum detection efficiency is also position-dependent via the inclusion of the optical physics. It is enhanced by $$\sim$$ 8% close to the SiPMs and suppressed by $$\sim$$ 2% in the adjacent corners with respect to the average value. Finally, we explore the energy threshold and resolution of the bottom ACS, considering the impact of its smaller crystals compared with the lateral walls.

The Compton Spectrometer and Imager (COSI) is a Compton telescope designed to survey the 0.2–5 MeV sky, consisting of a compact array of cross-strip germanium detectors. As part of its development, in 2016 COSI had a successful 46 day flight on board NASA’s Super Pressure Balloon platform. This was a precursor to the COSI Small Explorer (COSI-SMEX) satellite mission that will launch in 2027 into an equatorial low Earth (530 km) orbit. The observation of MeV gamma rays is dominated by background radiation, especially due to the activation of the detector materials induced by cosmic-ray interactions. Thus, background simulation and identification are crucial for the data analysis. Because the COSI-SMEX detectors will be similar to the ones used for the balloon flight, the balloon measurements provide an important tool for testing and cross-checking our background simulations for the upcoming space mission. In this work we perform Monte Carlo simulations of the background emission from the 2016 COSI balloon flight. Including a phenomenological shape correction, we obtain an agreement with the data at the 10%–20% level for energies between 0.1 and 1.6 MeV, and we successfully reproduce most of the activation lines induced by cosmic-ray interactions.

We present measurements of the afterglow signatures in NaI(Tl) and CsI(Tl) detector modules as part of the Background and Transient Observer (BTO) mission detector trade-study. BTO is a NASA Student Collaboration Project flying on the Compton Spectrometer and Imager (COSI) Small Explorer mission in 2027. The detectors utilized in this study are cylindrical in shape with a height and diameter of 5.1 cm and were read out by silicon photomultipliers (SiPMs). We conducted a radiation campaign at the HIMAC accelerator in Japan where the scintillators were irradiated with a 230 MeV/u helium beam (He beam) and 350 MeV/u carbon beam (C beam). We find that both the CsI and NaI scintillators exhibit afterglow signatures when irradiated with the C and He beams. The CsI crystal exhibits a stronger afterglow intensity with afterglow pulses occurring for an average 2.40 ms for C and 0.9 ms for He after the initial particle pulse. The duration of afterglow pulses in CsI is 8.6x and 5.6x the afterglow signal duration in NaI for C and He (0.28 ms and 0.16 ms, respectively). Although CsI has advantages such as a higher light yield and radiation hardness, the stronger afterglows in the CsI detector increase the complexity of the electronics and lead to a 7x larger dead time per afterglow event or a 3x higher energy threshold value. We use the measured dead times to predict the amount of observing time lost to afterglow-inducing events for an instrument like BTO in low Earth orbit. We simulate the background rates in a BTO-like orbit and find a total value of 114 counts/s for the full two-detector system. Based on the particle energies in the HIMAC experiment, we then determine that an event with sufficient energy to produce an afterglow signal occurs once every 70 s and 1.4 s in NaI and CsI detectors, respectively. Thus, we conclude that NaI is the better choice for the BTO mission.

Understanding the nature of the accretion disk, its interplay with the X-ray corona, and assessing black hole spin demographics remain open challenges in astrophysics. In this paper, we examine the predictions of the standard α-disk model, origin of the puzzling soft X-ray excess, and measure the black hole spin parameter by applying an updated high-density disk reflection model to the XMM-Newton/NuSTAR broadband (0.3-78 keV) X-ray spectra of a sample of 11 Type-1 AGN. Our Bayesian analysis confirms that a variable-density relativistic disk reflection model with a broken power-law emissivity profile can simultaneously fit the soft X-ray excess, broad iron K line emission, and Compton hump in 3 out of 11 AGN. For the remaining sources, a distinct warm Comptonization component is still required, which supports a hybrid origin for the soft X-ray excess. The measured temperature and optical depth of the warm corona span nearly the entire theoretically allowed range, with median values of 0.43_-0.18^+0.40 keV and 12.5_-3.9^+3.1, respectively. Our first systematic calculation of the disk-to-corona power transfer fraction reveals that the fraction of power released from the accretion disk into the hot corona spans a wide range, with a sample median of 0.68_-0.25^+0.25. The sample median values for the hot coronal plasma temperature and optical depth are 54_-12^+11 keV and 0.98_-0.28^+0.22, respectively. Finally, through both hard X-ray (3-78 keV) and broadband (0.3-78 keV) relativistic reflection spectroscopy, we systematically constrain the black hole spin parameter across the mass scales of log(M_ BH/M_⊙) ∼ 5.5-9.0, thereby increasing or refining the available spin measurements in the AGN population by ∼20

IGR J17091−3624 is the only black hole X-ray binary candidate—aside from the well-studied black hole system GRS 1915+105—observed to exhibit a wide range of structured variability patterns in its light curves. In 2025, the source underwent a “failed” outburst: it brightened in the hard state but did not transition to the soft state before returning to quiescence within a few weeks. During this period, IGR J17091−3624 was observed by multiple ground- and space-based facilities. Here, we present results from six pointed NuSTAR observations obtained during the outburst. None of the NuSTAR light curves showed the exotic variability classes typical of the soft state in this source; however, we detected, for the first time, strong dips in the count rate during one epoch, with a total duration of ∼4 ks as seen by NuSTAR. Through spectral and timing analysis of all six epochs, we investigate the hard-state spectral evolution and the nature of the dips. A clear evolution of the coronal properties with luminosity is observed over all six epochs, with clear signatures of relativistic disk reflection that remain largely unchanged across the first five epochs. The first five epochs also show a strong and stable quasiperiodic oscillation feature in the power spectra. The dips observed in Epoch 5 are consistent with partial obscuration by ionized material with a column density N _H ≈ 2.0 × 10 ^23 cm ^−2 . We discuss possible origins for this material and place constraints on the orbital parameters and distance of the system.